Various types of condition sensing devices in common use provide a control unit and a plurality of detector devices connected in parallel across a voltage source. Each detector unit comprises a pair of circuit elements connected in series across the power leads through a junction. At least one of the circuit elements is gradually variable in impedence with a change of condition to be measured or detected, causing a change in voltage at the junction. Means is provided at the control unit responsive to the change in voltage to actuate an alarm, or to control some other device. In one particular type of detector, at least one of said circuit elements is a photo-cell responsive by a decrease in resistance to a particular type of radiation, such as infra-red. A detector system of this type is shown in U.S. Pat. No. 3,122,638 issued Feb. 25, 1964, an improved form of detector element for use therewith is shown in U.S. Pat. No. 3,188,593, issued June 8, 1965, and an amplifier for use with such a system is shown in U.S. Pat. No. 3,725,660, issued Apr. 3, 1973.
One desired feature of such a detector system that has not been heretofore available is satisfactory means for providing an indication of broken or shorted leads to the remote detector elements. In detector devices utilizing photo-cells as described in the above mentioned patents, the cells normally have a very high impedance. Hence it is difficult or impossible to reliably check the continuity of the three leads to a remote detector, since the photo-cells are, in effect, an open circuit during standby conditions.
The object of this invention, therefore, is to provide an economical and reliable circuit for such a detector that is capable of indicating when any one of the three leads to a detector or a group of detectors connected in parallel is broken, shorted to another lead, or shorted to ground, by applying a supervisory voltage to the signal lead in such a manner that the supervisory voltage varies in proportion to variations in the supply voltage, so that the supervisory voltage remains proportional to the voltage produced at the signal lead by a detector in response to an external condition to be detected, and to the voltage to which the alarm actuating unit responds to produce an alarm, so that variations in supply voltage cannot cause a false alarm or a false indication of a broken circuit.